This invention relates to a screw grommet made of synthetic resin, and more particularly to a screw grommet which is so adapted that when it is used to fix on a mounting panel a varying part made of synthetic resin and, therefore, disposed to undergo thermal expansion or thermal shrinkage due to change of the atmospheric temperature, it will infallibly absorb a possible error of manufacture or fabrication imparted to the mounting panel, enable itself to be automatically slid to the center of a fitting hole bored in the mounting panel and, at the same time, infallibly absorb a variation possibly brought about in the position thereof relative to the mounting panel by thermal expansion or thermal shrinkage of the part of synthetic resin.
A conventional screw grommets is disclosed in U.S. Pat. No. 2,788,047.
This particular conventional grommet is a one-piece article of synthetic resin which, though not specifically illustrated, comprises an enlarged head part adapted to abut against the surface of a mounting panel, a shank part adapted to engage a fitting hole bored in the mounting panel, and a screw socket extending from the head part into the shank so as to permit insertion therein of a screw member. Actual use of this shaped article in the fixation of a given part to a mounting panel is attained by inserting the shank of the shaped article into a fitting hole bored in the mounting panel, aligning a throughhole bored in the part with the opening of the screw socket, and driving a screw member past the through hole into the screw socket thereby causing the wall of the shank to be radially expanded and brought into fast engagement with the edge of the fitting hole, with the result that the part is fastened to the mounting panel.
Generally the mounting panel is provided with a plurality of such fitting holes and the part is likewise provided with as many throughholes as the fitting holes. Where a spacing error due to manufacture or fabrication occurs between the positions of the fitting holes, and those of the through holes or a similar dimensional error between the diameters of the holes and those of the grommet shanks, it is frequently observed that conventional grommets cannot be fitted in a correct posture in the corresponding fitting holes but are instead fitted in an oblique posture therein without reference to the discrimination between the two sorts of errors mentioned above because these grommets are devoid of any means or structure capable of absorbing the error.
The conventional grommets, therefore, have had the disadvantage that once they are fitted obliquely in the fitting holes, the parts destined finally to be fixed on the mounting panel are fixed with very poor stability on the panel and, where correction is required, the work of refixation becomes highly complicated.
An improved grommet disclosed in U.S. Pat. No. 2,836,215, for example, has been proposed to solve the situation. Basically, this improved grommet is similar to the aforementioned conventional grommet in the sense that it is provided with an enlarged head part, a shank, and a screw socket extending from the head part into the shank to permit insertion therein of a screw member. Distinctly, however, this improved grommet is additionally provided with a pair of enaging pieces disposed on the opposite outer sides of the shank and raised upwardly along the length of the shank, so that the fast engagement of the grommet shank with a fitting hole in a mounting panel is accomplished by causing the upper edges of the engaging pieces to be abutted elastically against the edge of the fitting hole.
This improved grommet, therefore, is capable of thoroughly absorbing a dimensional error between the diameter of the fitting hole and that of the shank because the pair of engaging pieces raised upwardly as described above are allowed to be bent to some extent by virtue of the spaces defined by the inner surfaces of the engaging pieces and the outer surface of the shank. Since these engaging pieces are intended exclusively as means for the engagement of the grommet shank and the fitting hole, however, they are incapable of efficiently and infallibly absorbing the spacing error possibly occurring between the positions of the fitting holes and those of the through holes.
Where a spacing error occurs between the positions of the fitting holes and those of the through holes, therefore, the improved grommet is incapable of absorbing this spacing error and is therefore not fitted in a correct posture in the fitting hole, but is fitted in an oblique posture therein. The disadvantage of the conventional grommet that the part is fixed with poor stability on the mounting panel and, where a correction is required, the work of refixation itself becomes complicated is not solved by this improved grommet.
Particularly where the part given to be fixed on the mounting panel happens to be made of synthetic resin, a substance liable to undergo thermal expansion or thermal shrinkage due to change in the atmospheric temperature, it is undeniable that the part, after the fixation on the panel, has the possibility of being thermally expanded or thermally shrunken by change in the ambient temperature and consequently inducing relative variation between the positions of the fitting hole and the through hole. Since the improved grommet is likewise devoid of any means for effecting efficient absorption of the variation in the positions of the holes due to change in the ambient temperature, it has a disadvantage in that the fixation of the part is eventually deprived of stability.
Moreover, in the use of the screw grommet, it has been customary to ensure a waterproofing effect and shock-absorbing effect to the intervening space between the enlarged head part and the mounting panel by attaching a packing material made of sponge sheet, for example, to the lower surface side of the enlarged head part thereby interposing the packing material between the lower surface of the enlarged head part and the upper surface of the mounting panel. Thus, the former conventional grommet and the latter improved grommet have the common disadvantage that once they are fitted in an oblique posture in the fitting holes of the mounting panels, they cannot rely fully upon the waterproofing and shock-absorbing effect of the packing material because the packing material is compressed unevenly between the enlarged head part and the mounting panel.